This invention relates to a pole-zero error correction circuit. More particularly, it relates to an automatic pole-zero error correction circuit for use in an ionizing radiation spectroscopy system.
Spectroscopy systems are used for the measurement and analysis of electromagnetic spectra arising from the emission or absorption of radiant energy. Most radiation spectroscopy systems employ a radiation detector for receiving incident radiation so that a charge, proportional to the incident radiation energy, can be provided. This charge is directed to a charge sensitive preamplifier for forming a voltage pulse exhibiting a fast rise time with a long decay time typically on the order of 5 us-1000 us. Further analog amplification and filtering are performed upon this pulse to provide a properly amplitude scaled pulse with good signal-to-noise characteristics. The filtering, also known as pulse shaping, is typically implemented in the form of active high pass or differentiation filter stages followed by several active low pass filter stages. Collectively, the various stages could be called a pulse shaping network. A typical pulse shaping network forms a pulse having a semi-Guassian shapexe2x80x94one which resembles a bell.
Optimal performance of a spectroscopy system is best realized when the output of the pulse shaping network returns to baseline without extending for a long period of time above or below the baseline. If the output pulse of the pulse shaping network fails to return to baseline, an error is introduced into the system during the measurement of the next pulse which occur randomly in time and amplitude. The difference between the time constants of the preamplifier and high-pass filter stages of the spectroscopy system is the source of the error. Adjustment in the pulse shaping network by some type of pole-zero adjustment means is required to correct this error.
Some correction circuits have been introduced into spectroscopy systems which permit manual adjustment of the error. The device of U.S. Pat. No. 4,491,799 to Giardinelli is used with a manual adjustment system and permits the operator to verify that the proper manual adjustment has been made. In particular, the Giardinelli device employs a pair of LEDs for providing a visual indication of the value and polarity of a boxcar integrator output. If one of the LEDs illuminates, the operator knows that the baseline has either fallen below or has remained above the preferred null point. Manual adjustment is then implemented until both LEDs fail to illuminate indicating that the baseline is at the desired null point. The Giardinelli invention attempts to eliminate the need for reading an oscilloscope, but falls short of providing a system which can provide accurate, let alone automatic pole-zero cancellation. In particular, the Giardinelli invention is a completely analog circuit containing many inherent qualities which contribute to the exact error in which the invention is attempting to eliminate (i.e., phase shifts, time phase anomalies, temperature drifts and component tolerances). Further, there exists no means for calibrating the circuit to compensate for the error introduced by the circuit. Still further, the Giardinelli circuit uses a boxcar integrator for sampling the baseline of each pulse passing through the circuit and for providing an average value for the baseline based on all of the sampled pulses. But, since pulses are random in nature, in time and frequency, sample averaging is at best an approximation of the conditions passing through the circuit and do not represent truly ideal conditions needed for an accurate spectroscopy system. Essentially, low count scenarios will result in inaccurate results when using the Giardinelli device. Yet still further, the Giardinelli device requires an operator to make a manual adjustment, through the use of a potentiometer, to achieve proper compensation. Operator error also contributes to inaccurate results.
U.S. Pat. No. 4,866,400 to Britton et al. discloses an automatic pole-zero adjustment circuit for use in ionizing radiation spectroscopy systems and represents an improvement over circuits then known in the prior art such as the Giardinelli device. However, as will be discussed, the invention shown therein still fails to provide a pole-zero cancellation circuit which can accomplish its function without the use of un-needed additional components, as well as being a circuit which relies on pulse sample averaging for determining whether the baseline has returned to the desired null point. The Britton device uses an analog averager to form an average value of the over or under-shoot of a plurality of pulses. Thereafter, the ADC samples this averaged value. Once a plurality of pulses have passed therethrough, the value representing the average amplitude, outputted from the ADC, is directed to a control circuit which affects a command upon a pole-zero network coupled to the system. Unfortunately, such as discussed in Giardinelli, sample averaging is inherently deficient in that it does not represent true real time dynamic sampling. Britton also uses the additional analog-to-digital converter (above what is normally needed in a spectroscopy system) with the boxcar averager in its control circuitry, thereby dissipating additional power, as well as adding further cost and complexity to the circuit. Further, the Britton control circuitry is relying on a single average value of the sampled pulses and is not configured to accept and process continuous individual values passing through its circuitry. Accordingly, it is configured only to output a single control word in response to a single accepted sampled value.
An improved pole-zero error correction device/circuit is needed for use in ionizing radiation spectroscopy systems which can overcome the deficiencies in the prior art. The improved device should eliminate analog averaging and rely on the actual value of each sampled pulse (a pulse-by-pulse basis) for determining the extent of the pole-zero error. Further, the device should eliminate un-needed components which tend to dissipate additional power and impose additional cost and complexity to the system. Finally, the improved pole-zero device should automatically correct the error and then shutdown once the error has been corrected.
We have invented an improved pole-zero error correction circuit and method for correcting pole-zero error in an ionizing radiation spectroscopy system. Our circuit overcomes the deficiencies seen in the prior art and represents a major improvement to that which is known by those skilled in the art.
An automatic pole-zero error correction circuit couples to an ionizing radiation spectroscopy system. The circuit includes a pole-zero network coupled to the spectroscopy system as part of a larger feedback control system. The pole-zero network has a characteristic which varies in response to an input control signal received from a pole-zero compensation circuit. The pole-zero network performs an adjustment to the spectroscopy system to correct the pole-zero error within the spectroscopy system. A gated integrator extends the shaped pulses for a set duration permitting two samples to be converted by an analog-to-digital converter (ADC) along the slope of each integrated pulse passing through the spectroscopy system. The two conversion results taken from each pulse are directed to the pole-zero compensation circuit for generation of an appropriate correction signal for subsequent injection upon the pole-zero network. The correction signal is a digital algorithm representing a digital correction factor outputted from a digital controller of the pole-zero compensation circuit.
The novel approach of correcting pole-zero error within a spectroscopy systems, as seen herein, provides a much higher degree of accuracy for correction of the pole-zero error over those circuits, devices and methods known in the prior art. In particular, the present device can achieve superior results over those devices utilizing boxcar averaging, a method of analog sampling known to be susceptible to errors especially when the repetition rate of the pulses is low. Further, the device of the present invention, taking two samples along the trailing edge of the extended pulse waveform, is actually measuring the area of the pulse and not the amplitude, a method of measuring for pole-zero error not seen hereto before. The device of the present invention also utilizes a reduced component count which dissipates less power, saves space and reduces cost of manufacturing. Reduced component count, as compared to those systems in the prior art, is realized through the novel use of components already provided in the spectroscopy system.